1. Field of the Invention
This invention relates to FM modulators that perform signal modulation, in particular, heterodyne-type FM modulators designed to generate radio frequency signals.
2. Description of the Related Art
There have recently been requirements for FM modulators that are capable of generating FM signals in an even wider bandwidth than provided by conventional FM modulators. One candidate for such a FM modulator has utilized semiconductor lasers. As described in a book titled "Coherent Optical Fiber Communications", (authored by Okoshi, et al., published by Ohm-sha), on pages 132 to 134, these FM modulators make use of the semiconductor laser diode characteristics that the frequency of the output optical signal can be modulated according to injection current variation, as well as output signal intensity. In this modulation scheme, optical heterodyne technology is applied to enable an electric circuit to handle signals in a light frequency region in the order of several hundreds of THz (1 THz=10.sup.12 Hz). A book, "Coherent Light Communication," authored by Shimada and published by the Institute of Electronics, Information and Communications, describes an aspect of the optical heterodyne technology on pages 9 to 18.
A combination of these technologies can achieve a heterodyne-type FM modulator that can provide FM signals in a wider band than accomplished by the conventional FM modulator.
FIG. 1 shows a general structure of a prior art FM modulator. This FM modulator is provided with two light sources; a light source 101 for modulation and a light source 102 used as a local oscillator. An electric signal 104 is designed to be supplied to an input terminal 103 of the light source 101 for modulation. The light source 101 for modulation has a first light emitting element 106 and the light source 102 used as local oscillator has a second light emitting element 107. The light from the first light emitting element 106 is modulated by the input electric signal 104.
A modulated light signal 108 emerging from the light source 101 for modulation and a signal 109 output from the light source 102 for local oscillation are input to an optical coupler 111, whose output signal is emitted as a light signal 112. The light signal 112 is designed to be subjected to heterodyne detection through a light receiving element 113 and output from an output terminal 115 as an output signal 114.
Some transmission systems employing these FM modulators were disclosed, for example, in "Optical Super Wide-Band FM Modulation Scheme and Its Application to Multi-Channel AM Video Transmission Systems", by Kikushima, et al. in IOOC '95 PS2-7, published in 1995 and "Noise Characteristics of Wide-Band FM Modulators" by Ishiguro, et al., in 1996 Spring Convention record of Institute of Electronics, Information and Communication Engineers of Japan, No. B-1207. These conventional FM modulators have realized the generation of wide bandwidth FM signals. However, there has arisen a new problem that a parasitic intensity modulation component accompanying FM modulation of a semiconductor laser by applying a injection current variation may deteriorate the transmission quality. This problem is discussed in detail in "Distortions of Optical Video Transmission Systems Employing AM/FM Converters and its Countermeasures", by Kikushima et al. in Technical Report of Institute of Electronics, Information and Communication Engineers of Japan, No. OCS96-7 published in May, 1996.
According to the above-mentioned document, Kikushima, et al. attempted to adjust the power ratio of the light for local oscillation to the light for modulation in order to suppress intensity modulation components. In FM modulators to which this power ratio adjustment was applied, however, the adjustment was made so that the optical power for local oscillation would be higher and that for modulation lower. As a result, the power of an electric signal obtained by optical heterodyne detection decreases, which results in increased noise, that is, the noise characteristic is degraded.
There has been another proposed scheme, in which the intensity modulation components is suppressed by passing the heterodyne-detected signal through a high-pass filter. This scheme is based on a fact that the intensity modulation components of an FM-modulated semiconductor laser exist in a lower frequency band than the FM modulation components. Intensity modulation components in lower bands can be suppressed by passing the heterodyne-detected signal through the high-pass filter. However, this approach also has a problem that the distortion characteristic is deteriorated by the frequency-dependent delay of the high-pass filter. This deterioration of the distortion characteristic is described in "Theoretical Analysis of Group Delay Distortion in the Fiber-Optic Video Distribution System Employing Super Wide-band Optical FM Scheme", by Ishii et al. in Technical Report of Institute of Electronics, Information and Communication Engineers of Japan, No.OCS96-40 published in June, 1996. In addition to the above-mentioned problems, optical heterodyne type FM modulators generate another distortion component due to nonlinear characteristics of semiconductor laser diodes. To reduce this distortion, it is necessary to decrease the power of signals to be input to semiconductor laser diodes. However, a decrease in the input signal power also decreases frequency displacement of FM modulated signals, which results in deteriorated noise characteristics.
As stated in Japanese Patent Laid-Open Publication No.3-209927 for a heterodyne transmitter for which frequency conversion was conducted using an intermediate frequency for radio frequency signal conversion, an attempt was made as follows: an n-multiple wave generator unit was used to generate a signal of n times the intermediate frequency. The output signal phase for the n-multiple wave generator was adjusted by a phase adjusting unit. The amplitude for the phase adjusting unit output signal was adjusted by an amplitude adjusting unit. Then, the output signal for the amplitude adjusting unit was combined with an intermediate frequency signal in a combiner unit. The combined signal was routed to a frequency conversion unit where the signal was modulated by a local oscillation frequency. By the above configuration, it is possible to eliminate unnecessary components produced by the distortion from the mixed modulation of a local oscillation frequency signal and an intermediate frequency signal.
FIG. 2 shows a configuration of the heterodyne transmitter presented in the above-mentioned Japanese Patent Laid-Open Publication. This heterodyne transmitter consists of a signal processing unit 121 which processes an input intermediate frequency signal f.sub.IF, and a frequency conversion unit 123 where the frequency of the signal output from a combiner unit 122 in the signal processing unit 121 is modulated by a local oscillation frequency f.sub.LO. The signal processing unit 121 comprises an n-multiple wave generator unit 124 which generates a signal having n times the input intermediate signal frequency f.sub.IF, a phase adjusting unit 125 which adjusts the phase of the output signal from the n-multiple wave generator unit 124, an amplitude adjusting unit 126 which adjusts the amplitude of the output signal from the phase adjusting unit 125, and a combiner unit 122 where the output signal from the amplitude adjusting unit 126 is combined with the intermediate frequency signal f.sub.IF.
By frequency conversion of the output signal from the signal processing unit 121 with local oscillator output of frequency f.sub.LO in the frequency conversion unit 123, this heterodyne transmitter can obtain a signal with a radio frequency f.sub.LO .+-.f.sub.IF, without unnecessary frequency components produced by the distortion from the mixed modulation of the signal with a local oscillation frequency f.sub.LO and the signal with an intermediate frequency f.sub.IF.
FIG. 3 shows an another configuration for eliminating unnecessary amplitude modulation components, which was disclosed in Japanese Patent Laid-Open Publication No.05-188334. In this method, an optical intensity modulator 132 is used for applying inverse amplitude modulation to PSK- or FSK-modulated optical signal to eliminate unnecessary amplitude modulation components. In this configuration, a phase difference between the residual amplitude modulation component 136 and the digital signal 131 is detected by an analog multiplier, which consists of a mixer 137 and a low-pass filter 138, in order to control the modulation index of the inverse amplitude modulation. The amplitude of the digital signal 131 is controlled by a variable amplifier 139 so that the output of the analog multiplier is zero, thus eliminating unnecessary light amplitude modulation components.
In the conventional circuit shown in FIG. 2, unnecessary components caused by the nonlinearity of the heterodyne detector can be canceled. However, unnecessary intensity modulation components included in optical signals which are input to the optical heterodyne detector cannot be canceled.
The conventional circuit shown in FIG. 3 employs the light source for modulation 101 shown in FIG. 1 in order to cancel unnecessary intensity modulation components. Therefore, the loss is increased due to the additional loss of the optical intensity modulator, which decreases the level of the optical input to the light heterodyne detector. The decreased level leads to the noise characteristics degradation. Furthermore, due to intrinsic poor linearity of the optical intensity modulation, the distortion characteristic is greatly deteriorated. In addition, the configuration in FIG. 3 requires expensive components, such as an optical intensity modulator, and complicated control circuits.